Chemical sensors are generally known for use in a wide variety of areas such as medicine, scientific research, industrial applications and the like. Fiber optic and electrochemical approaches are generally known for use in situations where it is desired to detect and/or measure the concentration of a parameter at a remote location. Structures, properties, functions and operational details of fiber optic chemical sensors can be found in U.S. Pat. No. 4,577,109 to Hirschfeld, U.S. Pat. No. 4,785,814 to Kane, and U.S. Pat. No. 4,842,783 to Blaylock, as well as Seitz, "Chemical Sensors Based on Fiber Optics," Analytical Chemistry 56(1):16A-34A (1984), each of which is incorporated by reference herein.
Publications such as these generally illustrate that it is known to integrate a chemical sensor with a fiber optic waveguide, an electrochemical gas sensor or the like, in a manner such that the chemical sensor will interact with the analyte. This interaction results in a change in optical properties, which change is probed and detected through the fiber optic waveguide or the like. These optical properties of chemical sensor compositions typically involve changes in colors or in color intensities. In these types of systems, it is possible to detect particularly minute changes in the parameter or parameters being monitored in order to thereby provide especially sensitive remote monitoring capabilities. Chemical sensor compositions that are incorporated at the distal end of fiber optic sensors are often configured as membranes that are secured at the distal tip end of the waveguide device or optrode.
Gas sensors of this general type are useful in monitoring gas concentrations such as oxygen and carbon dioxide in bloodstreams and the like. Also, it is sometimes desirable to provide sensors that monitor other parameters such as pH. Ion concentrations can also be detected, such as potassium, sodium, calcium and metal ions.
A typical fiber optic sensor device positions the sensor material at a generally distal location with the assistance of one or more types of support means. Support means must be such as to permit interaction between a parameter-sensitive indicator--e.g., a fluorescent dye or the like--and the substance being subjected to monitoring, measurement and/or detection. Known approaches in this regard include the use of permeable membranes and composites incorporating micro-encapsulation.
One example of such an approach is found in U.S. Pat. No. RE 31,879 to Lubbers et al. That patent discloses a device wherein indicator material is provided in solution form and separated from the external environment by a membrane. This approach has been followed more recently by a number of others working in the field as well.
An alternative approach has been to attach an indicator composition to the tip of an optical fiber using a silanization technique. This method generally involves silanization of beads of porous glass, followed by covalent bonding of the indicator material to functional groups extending from the glass beads (generally through a siloxane linkage, as shown, for example, in U.S. Pat. No. 5,354,825 to Klainer et al.), in turn followed by attachment of the glass beads to the optical fiber. This technique, however, suffers from several drawbacks. For example, attachment of the glass beads to the fiber can be difficult because of the small size of the beads and the ease with which the pores in the glass are occluded. Additionally, these types of sensors cannot be extensively exposed to aqueous solutions, as glass dissolution and indicator leaching will occur through hydrolysis of siloxane linkages.
Still another technique involves direct bonding of photoactive polymers to the tip of an optical fiber, as described in U.S. Pat. No. 5,354,825 to Klainer et al., referenced above. While this method is effective, there nevertheless remains a need in the art for alternative methods of providing indicator materials on a fiber optic sensor device which are reliable, highly sensitive, simple to carry out and readily scaled up for purposes of manufacture.
The present invention provides such a method, and involves the use of an adhesive layer for affixing an indicator composition (sometimes referred to herein as "sensing chemistry") to the distal end of a fiber optic sensor. The method involves simple coating and curing procedures, and is thus straightforward to carry out. Additionally, the novel method enables control of sensor tip geometry, provides a protective layer for the fiber tip, and enables spatial partitioning of materials incorporated into the sensing area, e.g., indicator and reference dyes or the like. As will be explained in detail herein, the method also enables the use of more indicator at the sensor tip by increasing the available surface area to which an indicator-containing composition can bind, and provides for better adhesion of the indicator composition to the sensor surface, through either chemical bonding, mechanical adhesion, or both. A very reliable and highly sensitive sensor device is thus provided in which the likelihood of leaching and/or delamination of the indicator composition is minimized.